The increasing added mass of a bouncing ball

Abstract

Abstract A sphere moving through a fluid has a larger inertia than one moving through a vacuum. This increase in the inertia from the fluid is known as the added mass. The added mass has a large effect on the acceleration of the sphere if the sphere has a low density, and if the sphere is near a solid boundary. These effects are studied here by measuring the acceleration due to gravity of beach balls, in air, as they bounce vertically on the floor. The measured accelerations are smaller when the ball is closer to the floor because the added mass is larger there. These measurements are used to calculate the added mass of a sphere as a function of distance from the floor. The results collapse to a curve that is independent of ball size, and generally agrees with the predictions for an ideal fluid.